Is it meaningful to quantify vacancy concentrations of nanolamellar (Ti,Al)N thin films based on laser-assisted atom probe data?

Abstract

Recently, it was proposed to quantify vacancy concentrations based on laser-assisted atom probe tomography (APT) data for a (Ti,Al)N film with a nanolamellar architecture, grown by chemical vapor deposition. To determine, whether vacancy concentrations can be reliably estimated based on APT data, we systematically compare measurements with an ultraviolet (UV) as well as a green laser to ion beam analysis data. First, monolithic TiN and AlN films, which are stoichiometric within the measurement uncertainties of ion beam analysis, are investigated. In case of TiN an electric field strength of 39.4 V nm−1 and a nitrogen content of 49.0 at.%, consistent with ion beam analysis data, is obtained with both lasers, when using pulse energies of 5 pJ (UV laser) and 0.1 nJ (green laser). However, significant differences can be observed for monolithic AlN as nitrogen contents from 41.9 to 35.8 at.% and 48.4 to 41.4 at.% are measured, depending on variations of the pulse energies of the UV and green laser, respectively. In order to mimic a nanolamellar architecture for correlative compositional analysis by ion beam analysis and laser-assisted APT, a TiN/AlN/TiN trilayer film is synthesized and significant deviations with respect to the nitrogen content are evident for the AlN layer. While the average nitrogen concentration from ion beam analysis is 51 at.%, nitrogen contents from laser-assisted APT data are only 38 at.% (UV laser) or 41 at.% (green laser). Hence, the nitrogen content of the AlN layer is underestimated from laser-assisted APT data by at least 10 and up to 13 at.% and this significant discrepancy cannot be explained by the measurement uncertainties. Thus, the here presented data emphasize that it is not meaningful to quantify vacancy concentrations of nanolamellar (Ti,Al)N thin films solely based on atom probe data, since the measurement accuracy depends on the field evaporation conditions.